For spectacular wildlife viewing there is nothing quite like the annual bison breeding season in Yellowstone National Park’s Hayden Valley in late July and August.
“That is an absolutely amazing place to view bison,” said Chris Geremia, the park’s senior bison biologist.
Now the national mammal, bison are hulking remnants of a species that once spread across the United States from Alaska to Mexico and east of the Mississippi River. Yellowstone represents a great success story for the animals. Once reduced to only 25 bison, through human intervention and protection the herd now numbers around 4,600.
The bison’s recovery in Yellowstone is an ecological triumph, but some scientists have in the past — as well as more recently — argued that the park is suffering under the big animals’ near-constant presence. Grasslands are being overgrazed, they say, trees like willow and cottonwood have disappeared because of browsing. Large winter migrations of bison and elk out of the park — which have resulted in controversial hunts and shipment to slaughter of captured bison — are due to a lack of forage, they contend.
One of the scientists behind such recent claims is Jeff Mosley, a professor of rangeland ecology and management at Montana State University. Mosley helped write and research parts of four studies published late last year that examined the condition of Yellowstone’s Northern Range — a 380,000-acre area of the park that supports the majority of bison, elk and pronghorn antelope.
“There was a perception that the range had degraded some since it was once looked at,” Mosley said.
That last detailed look — the latest of several — was conducted in 2002. That was when the National Research Council published a review of the Northern Range’s ecological health.
“The report concluded that degraded rangelands existed in YNP because of excessive grazing and browsing by wild ungulates, but the NRC report expressed optimism that predation by wolves (reintroduced into the Northern Range in 1995 to 1996) would eventually be sufficient to regulate ungulate populations inside YNP, thereby enabling degraded rangelands to recover,” Mosley wrote in his study, “History and Status of Wild Ungulate Populations on the Northern Yellowstone Range,” published in the December issue of the journal “Rangelands.”
Mosley was one of a crew of scientists to follow up on that 2002 study with a fresh look. One of the main conclusions was that Yellowstone needs to reduce its ungulate population — both bison and elk.
Acknowledging that such reductions in two popular species will ignite a firestorm of public protests, Mosley said the studies’ authors have been quietly offering to work with the National Park Service and Department of Interior to share their expertise in a collaborative way to come up with some fresh ideas.
“We’re not trying to stir up a controversy,” he said.
But mention Yellowstone and bison and the word controversy is usually close behind.
Geremia, the park’s bison biologist, explained in a recent Facebook live broadcast that Yellowstone’s research has shown that the short grasses tourists encounter in the park shouldn’t be seen as unhealthy. He compared the short grass to a mowed lawn. Lots of grass has already been removed by elk, bison and other species. That grass has just been converted into a different part of the ecosystem.
“Over the course of the growing season you produce more grass, more food for herbivores,” he said in a follow-up interview.
“That’s a very important thing.”
Studies have shown that the grazed grasses are actually healthier than those left alone, protected from grazing by fences, he said. The reasons are several. One is that shorter plants lose less moisture as their pores open to take in carbon dioxide, he explained. Another is that the bison deposit urine and feces that fertilize the grass, giving the soil necessary nutrients like nitrogen.
Soil samples show that the organic process is functioning properly in the park, Geremia said. Many microbes thrive in areas of intensive grazing.
Grassland production was found to be “resilient to the relatively high rates of prolonged grazing by the bison-dominant community and did not reduce (aboveground net primary production) below paired, ungrazed conditions,” according to a 2016 study authored by Douglas Frank, of Syracuse University, along with Yellowstone scientists Rick Wallen and P.J. White. “However, YNP grassland should continue to be monitored if such high rates of herbivory continue.”
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The differences of opinion highlight how rangeland scientists view a landscape versus wildlife biologists. Yet Mosley said his and his colleagues’ research was based on methods endorsed by the Park Service and other Department of Interior agencies.
“We’re not focusing on the use of the land,” he said. “We’re focusing on the health of the land. So it’s not biased by the use.”
But Geremia said range scientists are looking to manage for “hypothetical climax” species, the idea of vegetative succession. Starting from bare soil, certain plants will grow, followed by others that eventually lead to a climax community of plants.
“What we’ve learned now is that initial model (dating back to the 1930s) is kind of wrong,” Geremia said.
Instead of plant communities moving in a specific direction, they can move in many different directions, he said. “It’s a lot messier than that initial model.
“We expect those (plant) communities to appear and disappear,” he said. “There’s a natural flow that will support a more biologically diverse system.”
In Yellowstone, all of the functional plant species are present, Geremia said, from deep-rooted trees like willow and aspen to shallow-rooted species like grasses. Even though comparisons of old photos to recent ones show the disappearance of species like willow and aspen in the “Rangelands” studies, Geremia said the change isn’t bad for the ecosystem.
“There’s always disagreement on trying to identify what is appropriate or what is good and it depends on what you are comparing it to,” he said. “If your vision of what healthy is is different, even if you are using the same (scientific) tools, you will have a different answer.”
Bison advocacy groups like Buffalo Field Campaign argue that managing wild animals within a confined space like Yellowstone is unnatural and unrealistic. Instead, they support more room for the animals to roam outside the park’s confines on nearby public lands.
“The overwhelming evidence and research to date can find no particular degradation of Yellowstone's rangelands,” wrote conservationist George Wuerthner in an opinion article published on ForWolves.com. “This is not to suggest that these studies don't document some effects to the vegetation as a result of ungulate herbivory. They do. To find changes in the landscape but no serious degradation are not mutually exclusive.”
Gardiner-area rancher Frank Rigler disagrees. He sees the park and some of the surrounding elk winter range as degraded by wildlife.
“It’s shot now,” he said of the Dome Mountain Wildlife Management Area north of the park, which he started with his wife in 1986 by selling the land to Montana Fish, Wildlife & Parks. “It’s so overgrazed. Since they planted the wolves the elk never leave. It’s gone from blue bunch wheat grass to cheatgrass.
“A lot of this ground will never recover.”
Elk and bison numbers have fluctuated dramatically over the past 150 years. Elk populations topped out at 19,000 before being reduced following the reintroduction of wolves in the 1990s. Elk now number around 5,800, but more elk migrate out of the park in winter than in years past, about 75 percent.
Some elk, finding homes on irrigated agricultural land where public hunting isn’t allowed, have chosen not to return to the park. Others may be displaced because there are more bison, a dominant species, Mosley said.
The scientists writing for “Rangeland” said more elk outside of Yellowstone is likely to increase the spread of brucellosis, a disease carried by elk and bison that can cause pregnant cattle to abort. Via elk the scientists predicted brucellosis will expand to the states of Utah and Colorado in the next 20 years.
“In many ways, the issues of brucellosis and elk are more far reaching,” Mosley said.